ER3+ doped boro-tellurite glasses for 1.5 μm broadband amplification

ABSTRACT

A tellurite-based glass composition for use in EDFAs exhibits higher phonon energy without sacrificing optical, thermal or chemical durability properties. The introduction of boron oxide (B 2 O 3 ) into the Er 3+ -doped tellurite glasses increases the phonon energy from typically 785 cm −1  up to 1335 cm −1 . The inclusion of additional glass components such as Al 2 O 3  has been shown to enhance the thermal stability and particularly the chemical durability of the boro-tellurite glasses. Er:Yb codoping of the glass does further enhance its gain characteristics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to Er³⁺ doped tellurite glasses and morespecifically to Er³⁺ doped boro-tellurite glasses with increased phononenergy for 1.5 μm broadband amplification.

2. Description of the Related Art

Optical amplifiers are considered enabling components for bandwidthexpansion in fiber optic communications systems. In particular, silicaglass erbium doped fiber amplifiers (EDFA) exhibit many desirableattributes including high gain, low noise, negligible crosstalk andintermodulation distortion, bit-rate transparency, and polarizationinsensitive gain. These properties make optical fiber amplifierssuperior to semiconductor devices as amplifiers in fiber optic systems.Moreover, fiber-based amplifiers do not require conversion fromelectrical energy to photon energy. In a communications system of anysignificant size, there is typically a distribution network thatincludes long communication paths and nodes where the network branches.In such a network, amplifiers are required in order to maintain theamplitude of the signal and the integrity of any data in route between asource and destination. To function properly, the amplifiers mustexhibit high small signal gains and/or high output saturation powersover a desired bandwidth. One drawback of silica EDFAs is their limited30 nm bandwidth, which limits the transmission capacity of WDM systems.

Tellurite glasses provide a broad bandwidth of over 70 nm and thus havereceived considerable attention for use in EDFAs. See A. Mori et al “1.5μm Broadband Amplification By Tellurite-Based EDFAs,” Technical Digestof Conf. Optical Fibe-Comm. 1997 (OFC′97), Feb 16-21, 1997 and Y. Ohishiet al. “Gain Characteristics of Tellurite-Based Erbium-Doped FiberAmplifiers for 1.5 μm Broadband Amplification” Opt. Lett., vol. 23, no.4, 1998, p. 274.

To amplify a 1.5 μm signal, EDFAs can be optically pumped at 1480 nm orat 980 nm as shown in the energy level diagram 10 of Er³⁺, FIG. 1.Pumping at 1480 nm is typically used for high power EDFAs because theground state absorption to the ⁴I_(13/2) energy level has a highabsorption cross-section relative to the ⁴I_(11/2) energy level.Unfortunately, this scheme does not provide full population inversion orgood SNR and is not adequate for many EDFA applications.

980 nm optical pumping provides good SNR and low cost but the smallsignal gain is significantly less than what is achieved with 1480 nmpumping for erbium doped low phonon energy glass fibers, such asfluorite glass fiber and tellurite glass fiber. Tellurite glassescontain heavy elements, which translates into small phonon energy(typically between 680 and 785 cm⁻¹) as compared to silicate glasseswhich present high phonon energy (typically around 1100 cm⁻¹). Phononenergy has a strong influence on the lifetimes of the different excitedstates of Er³⁺ because the relaxation between levels is dominated bymultiphonon processes. The larger the number of phonons involved, thesmaller the probability of relaxation to the lower energy level, and thelonger the lifetime of a given excited state, for instance ⁴I_(11/2) oferbium ions.

With 980 nm pumping the level ⁴I_(11/2) gets populated first, and thenthrough phonon-assisted relaxation the lower level ⁴I_(13/2) getspopulated. Gain is achieved through transition between the levels⁴I_(13/2) and ⁴I_(15/2). For optimal operation, the lifetime of thelevel ⁴I_(11/2) should be as short as possible. Otherwise, excited stateabsorption processes from the level ⁴I_(11/2) to higher energy excitedstates such as ⁴F_(7/2) will occur and reduce the gain at 1550 nm.Consequently, the low phonon energy of tellurite glass creates longerlifetimes, which in turn reduces small signal gain when pumped with 980nm laser diode.

Y. G. Choi et al, “Enhanced ⁴I_(11/2)→⁴I_(13/2) Transition Rate inEr³⁺/Ce³⁺-Codoped Tellurite Glasses,” Electron. Lett. Vol. 35, no. 20,1999, p. 1765 proposed Ce³⁺-codoping to enhance the 980 nm pumpingefficiency through the non-radiative energy transfer Er³⁺:⁴I_(11/2),Ce³⁺:²F_(5/2)→Er³⁺:⁴I_(13/2), Ce³⁺: ²F_(7/2). The co-doping provides anadditional channel for the relaxation ⁴I_(11/2→) ⁴I_(13/2) in theEr³⁺-doped tellurite glasses which shortens the lifetime of the⁴I_(11/2) level and enhances the population accumulation in the⁴I_(13/2) level and the 980 nm pumping efficiency.

A more effective approach would be to increase the phonon energy of thetellurite glass without sacrificing the glass' optical, thermalstability or chemical durability properties.

SUMMARY OF THE INVENTION

The present invention provides a tellurite-based glass composition foruse in EDFAs that exhibits higher phonon energy without sacrificingoptical, thermal stability or chemical durability properties.

This is accomplished by introducing boron oxide (B₂O₃), which has aphonon energy up to 1335 cm⁻¹, into the Er³⁺-doped tellurite glasses.The introduction of B₂O₃ increases the phonon energy of the host glassand the multiphonon relaxation rate of the ⁴I_(11/2)→⁴I_(13/2)transition, which enhances the population accumulation in the ⁴I_(13/2)level and the 980 nm pumping efficiency. The inclusion of additionalglass components such as Al₂O₃ has been shown to enhance the thermal,stability and particularly the chemical durability of the boro-telluriteglasses. Er:Yb codoping of the glass will further enhance its pumpefficiency

In one embodiment, the boro-tellurite glass composition for the fibercore includes the following ingredients: a glass network former TeO₂from 50 to 70 mole percent, B₂O₃ from 5 to 22 mole percent, A₂O₃ from 5to 18 mole percent, a glass network modifier R₂O from 5 to 25 molepercent, a glass network modifier MO from 0 to 15 mole percent, GeO₂from 0 to 7 mole percent and rare-earth dopant L₂O₃ from 0.25 to 10weight percent wherein R₂O is selected from oxides Li₂O, Na₂O, K₂O andmixtures thereof, MO is selected from oxides MgO, CaO, BaO, ZnO andmixtures thereof, A₂O₃ is selected from Al₂O₃, Y₂O₃ and mixturesthereof, and rare-earth dopant L₂O₃ is selected from rare earth oxidesEr₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixtures thereof.The cladding glass has a similar composition absent the rare-earthdopants.

In another embodiment, the boro-tellurite glass composition for thefiber core includes the following ingredients: a glass network formerTeO₂ from 55 to 65 mole percent, B₂O₃ from 10 to 20 mole percent, A₂O₃from 7 to 15 mole percent, a glass network modifier R₂O from 10 to 20mole percent, a glass network modifier MO from 0 to 10 mole percent,GeO₂ from 0 to 5 mole percent and rare-earth dopant L₂O₃ from 0.25 to 6weight percent. In one embodiment, the glass comprises Al₂O₃ (A₂O₃) from7 to 15 mole percent and Na₂O (R₂O) from 10-20 percent. In anotherembodiment, the glass comprises Al₂O₃ from 10 to 15 mole percent. Theglass may be doped with, for example, 0.25 to 3 wt. % percent Er₂O₃,0.25 to 5 wt. % of an Er₂O₃ and Yb₂O₃ mixture, 0.25 to 5 wt. % each ofEr₂O₃ and Yb₂O₃, or approximately 0.25 to 3 wt. % of Tm₂O₃.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, as described above, is the energy level diagram of Er³⁺ intellurite glass;

FIGS. 2 through 4 provide boro-tellurite glass compositions inaccordance with the present invention;

FIG. 5 is a diagram of a boro-tellurite based EDFA;

FIGS. 6 and 7 are differential scanning calorimetry (DSC) curvesillustrating the thermal stability of the boro-tellurite glass;

FIGS. 8 through 10 are plots of weight loss per unit of surfaceillustrating the chemical durability of the boro-tellurite glass;

FIGS. 11 through 14 are transmission spectra illustrating the increasedphonon energy of the boro-tellurite glass; and

FIGS. 15 and 16 are gain curves and gain spectra illustrating theenhanced gain of a boro-tellurite glass fiber.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a tellurite-based glass composition foruse in EDFAs that exhibits higher phonon energy without sacrificingoptical, thermal stability or chemical durability properties. Boronoxide (B₂O₃), which has a phonon energy up to 1335 cm⁻¹, is introducedinto the Er³⁺-doped tellurite glasses. The introduction of B₂O₃increases the phonon energy of the host glass and the multiphononrelaxation rate of the ⁴I_(11/2)→⁴I_(13/2) transition, which enhancesthe population accumulation in the ⁴I_(13/2) level and the 980 nmpumping efficiency. The inclusion of additional glass components such asAl₂O₃ has been shown to enhance the thermal stability and particularlythe chemical durability of the boro-tellurite glasses.

In glass compositions, the glass network former, modifier and otherelements are typically specified in mole % because the glass structureis related with the mole % of every element in the glass. The dopantsare typically specified in weight % because the doping concentration interm of ions per volume, e.g., ions per cubic centimeters, can bereadily derived and is critical information for photonic and opticalrelated applications.

Boro-Tellurite Glass Composition

As shown in FIG. 2, the boro-tellurite glass composition 20 for thefiber core includes a glass network former TeO₂ from 50 to 70 molepercent. TeO₂ concentrations in excess of 70 mole percent produceglasses with a strong tendency to crystallize. B₂O₃ from 5 to 22 molepercent is introduced into the tellurite glass to raise the phononenergy of the lattice. Concentrations in excess of 22 mole percent tendto cause phase separation in the glass. The glass includes A₂O₃ (Al₂O₃,Y₂O₃ and mixtures thereof) from 5 to 18 mole percent, to increase theglass transition temperature and improve thermal stability andparticularly chemical durability. When the content of A₂O₃ exceeds 18mole percent, the melting temperature of glass becomes too high anddecomposition and crystallization could occur. The glass compositionfurther includes network modifiers R₂O (oxides Li₂O, Na₂O, K₂O andmixtures thereof) from 5 to 25 mole percent and MO (oxides MgO, CaO,BaO, ZnO and mixtures thereof) from 0 to 15 mole percent. The networkmodifiers are needed to obtain a stable tellurite glass but tend todeteriorate its chemical durability and lead to crystallizationappearance when present at elevated concentrations. GeO₂ from 0 to 7mole percent may be added to increase the glass transition temperatureand refractive index and improve thermal stability but GeO₂ is anexpensive material. Finally, the core glass is doped with a rare-earthdopant L₂O₃ from 0.25 to 10 weight percent selected from rare earthoxides Er₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixturesthereof. The cladding glass has a similar composition absent therare-earth dopants. The introduction of Al₂O₃ into the glass has beenfound to be particularly effective at improving chemical durability.

As shown in FIG. 3, a boro-tellurite glass composition 30 for the fibercore includes the following ingredients: a glass network former TeO₂from 55 to 65 mole percent, B₂O₃ from 10 to 20 mole percent, A₂O₃ from 7to 15 mole percent, a glass network modifier R₂O from 10 to 20 molepercent, a glass network modifier MO from 0 to 10 mole percent, GeO₂from 0 to 5 mole percent and rare-earth dopant L₂O₃ from 0.25 to 6weight percent. In one embodiment, the glass comprises Al₂O₃ (A₂O₃) from7 to 15 mole percent and Na₂O (R₂O) from 10-20 percent. In anotherembodiment, the glass comprises Al₂O₃ from 10 to 15 mole percent. Theglass may be doped with, for example, 0.25 to 3 wt. % percent Er₂O₃,0.25 to 5 wt. % of an Er₂O₃ and Yb₂O₃ mixture, 0.25 to 5 wt. % each ofEr₂O₃ and Yb2O₃, or approximately 0.25 to 3 wt. % of Tm₂O₃.

As shown in FIG. 4, a boro-tellurite glass composition 40 for the fibercore includes the following ingredients: a glass network former ofapproximately 60 mole percent TeO₂, approximately 15 mole percent B₂O₃,approximately 10 mole percent Al₂O₃, and approximately 15 mole percentNa₂O. The glass may be doped with, for example, 0.25 to 3 wt. % percentEr₂O₃, 0.25 to 5 wt. % of an Er₂O₃ and Yb₂O₃ mixture, 0.25 to 5 wt. %each of Er₂O₃ and Yb₂O₃, or approximately 0.25 to 3 wt. % of Tm₂O₃.

As previously described, in typical tellurite glass the low phononenergy causes the lifetime of the ⁴I_(11/2) level of erbium ions to berelatively long, which in turn reduces small signal gain. In addition,the long lifetime lowers the transfer efficiency from Yb ions to Er ionsso co-doping is not beneficial, hence not used in known telluriteglasses.

The introduction of boron oxide into the tellurite glass increases thephonon energy, which, as described above, has a direct impact on smallsignal gain. In addition, the higher phonon energy reduces the lifetimeof the ⁴I_(11/2) level. This shortening of the lifetime reduces backenergy transfer from Er to Yb ions and makes the Er:Yb codopingbeneficial. Thus in certain cases the glass is co-doped with Er:Yb toincrease the pump efficiency of the glass, fiber and EDFA.

Co-doping with ytterbium enhances population inversion of the erbium⁴I_(13/2) metastable state. The Yb³⁺ excited states ²F_(5/2) are pumpedfrom the Yb³⁺ ²F_(7/2) ground state with the same pump wavelength thatis used to excite upward transitions from the erbium ground state⁴I_(15/2). Energy levels of the excited ytterbium ²F_(5/2) statecoincide with energy levels of the erbium ⁴I_(11/2) state permittingenergy transfer (i.e. electron transfer) from the pumped ytterbium²F_(5/2) state to the erbium ⁴I_(11/2) state. Thus, pumping ytterbiumionic energy states provides a mechanism for populating the metastableerbium ⁴I_(13/2) state, permitting even higher levels of populationinversion and more stimulated emission than with erbium doping alone.

Ytterbium ions exhibit not only a large absorption cross-section butalso a broad absorption band between 900 and 1100 nm. Furthermore, thelarge spectral overlap between Yb³⁺ emission (²F_(7/2)-²F_(5/2)) andEr³⁺ absorption (⁴I_(15/2)-⁴I_(11/2)) results in an efficient resonantenergy transfer from the Yb³⁺ to the Er³⁺, exciting the ⁴I_(11/2) level.The energy transfer mechanism in an Yb³⁺/Er³⁺ co-doped system is similarto that for cooperative upconversion processes in an Er³⁺ doped system.However, interactions are between Yb³⁺ (donor) and Er³⁺ (acceptor) ionsinstead of between two excited Er³⁺ ions. Thus, in one embodiment thepresent invention utilizes Er:Yb co-doped boro-tellurite glass dopedwith 0.25 to 5 weight percent of an Er₂O₃ and Yb₂O₃ mixture. Typically,this glass is doped with 0.25-3 weight percent of Er₂O₃ and 0.25-3weight percent of Yb₂O₃.

As shown in FIG. 5, a boro-tellurite EDFA 50 includes a boro-telluriteactive fiber 52 of the type just described and a 980 nm optical pump 54.The active fiber is coupled between a pair of input and output fibers,53 a and 53 b, typically passive double-clad silica fiber. Coupling ofthe signal between fibers can be achieved using free-space optics or byfusion splicing. Optical pump 54 can be either a single-mode or amulti-mode pump and is coupled into the active fiber using a pumpcoupler 55 such as a WDM, a side-coupler such as Goldberg's V-groove asdescribed in U.S. Pat. No. 5,854,865 or by using a total internalreflection (TIR) coupler as described in co-pending U.S. patentapplication Ser. No. 09/943,257 entitled “Total Internal Reflection(TIR) Coupler and Method for Side-Coupling Pump Light into a Fiber”,which is hereby incorporated by reference. The optical pump excites theionic rare-earth dopants in the core of the fiber to produce stimulatedemission and amplification of an input signal propagating through thefiber. Using the boro-tellurite fiber of the present invention, the EDFAprovides a moderate amount of gain over a wide bandwidth.

Experimental Procedure

The development of the boro-tellurite glasses provided in FIGS. 2, 3 and4 are the result of considerable experimentation and analysis todetermine appropriate compositions that not only increase phonon energybut do so without reducing the bandwidth or deteriorating the optical,thermal or chemical durability properties of the glass.

The different glass compositions were prepared according to thefollowing procedure: high-purity oxides (99.999% and 99.99% pure) wereweighed according to desired oxide molar percentages and mixed. Eachmixture of powders was heated in a furnace at temperatures ranging from700° C. to 800° C. depending on the melting properties of eachcomposition. The melted bath (or glass, or mixture) was then kept undera flow at 10 LPM (liter per minute) of nitrogen gas. This treatmentremoves hydroxyl impurities (OH⁻) from the glass, which are known toreduce the light-emitting properties of the Er ions. After thistreatment, the melts were cast into moulds preheated at the glasstransition temperature of each composition and the solids were annealedat this temperature for two hours before being cooled down slowly toroom temperature over a period of 15 hours.

The determination of the glass transition temperature for eachcomposition was carried out by differential scanning calorimetry. Forthese experiments, glasses were first reduced to powder and placed intoalumina crucibles and heated at a rate of 10° C./min from roomtemperature to 800° C. under a flow of nitrogen gas at 0.1 LPM. Othertemperatures, including the onset crystallization temperature Tx, thecrystallization peak temperature Tc, and the melting point temperatureTm were determined following the same procedure.

To test the chemical durability of each glass composition, samples withdimensions of approximately 4×16×24 mm were cut, polished, and weighedcarefully. Then the samples were immersed in boiling water for fixedtime periods and carefully weighed again between successive immersions.An important weight loss (measured in units of mg/mm²) followingimmersion in boiling water is indicative of poor chemical durability andvice versa.

Other characterization experiments included density measurements inwhich the volume of each sample was determined by immersing the samplesin carbon tetrachloride, optical spectroscopy in the UV, visible andnear infra-red, and refractive index measurements using amultiwavelength prism coupler. For the determination of phonon energiesinfrared spectroscopy was performed. For these experiments, 1 mg ofglass powder was mixed with 150 mg of dried KBr and the mixture wasformed into a flat pellet by compression. During the experiments, thespectrophotometer was purged by dried air.

For the measurement of the rare-earth emission spectrum and fluorescencelifetime, 300 μm-thick samples with polished facets were prepared.Emission spectra were recorded with a spectrometer while the sampleswere pumped at 980 nm using a cw Ti:sapphire laser. Absolute values ofthe absorption and emission cross sections were calculated usingMcCumber theory. The fluorescence lifetime of Er³⁺ was determined fromthe measured fluorescence decay curve of the ⁴I_(13/2)→⁴I_(15/2)transition.

Experimental Results

Table 1 provides a list of glasses with their respective compositionthat were fabricated and tested. In this example, the Er₂O₃concentration was fixed at one weight percent of the total weight of theglass. For comparison, a glass containing tungsten oxide was alsoprepared.

When the Na₂O concentration is decreased in the boro-tellurite glasses,especially when Na₂O is replaced by Al₂O₃, their color changes fromyellow to pink. Since the glasses contain erbium ions, and since theseions are known to confer a pink color to a transparent glass matrix, itcan be deduced that the decrease of Na₂O, especially when it is replacedby Al₂O₃, shifts the UV absorption edge towards the shorter wavelength.

TABLE 1 Glass names and compositions. Glass Composition (% mol) 25Na 25Na₂O—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 5Te 20 Na₂O—15 B₂O₃—65 TeO₂—1_(WT)Er₂O₃ 5Ge 20 Na₂O—5 GeO₂—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 7Ge 18 Na₂O—7GeO₂—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 5Ge2Te 18 Na₂O—5 GeO₂—15 B₂O₃—62TeO₂—1_(WT) Er₂O₃ 5Al 20 Na₂O—5 Al₂O₃—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 7Al18 Na₂O—7 Al₂O₃ 15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 10Al 15 Na₂O—10 Al₂O₃—15B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 5A12Te 18 Na₂O—5 Al₂O₃—15 B₂O₃—62 TeO₂—1_(WT)Er₂O₃ 5Al 5Ge 15 Na₂O—5 Al₂O₃—5 GeO₂—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 10Al5Ge 10 Na₂O—10 Al₂O₃—5 GeO₂—15 B₂O₃—60 TeO₂—1_(WT) Er₂O₃ 15K25W 15K₂O—25 WO₃—60 TeO₂—1_(WT) Er₂O₃Thermal Properties

An important characteristic that defines a good glass is its resistanceto crystallization. Crystallization can occur when the glass is heatedabove its glass transition temperature and leads to an exothermic peakin a differential scanning calorimetry (DSC) curve. Such DSC curves areshown in FIGS. 6 and 7.

In FIG. 6, the curves 60, 61, 62 and 63 correspond respectively to theglasses 25Na, 5Te, 5Al, and 5Ge listed in Table 1. The first decrease inheat flow 65 corresponds to the glass transition temperature. Thisdecrease is followed by an exothermic peak 66 that is indicative ofcrystallization. All the curves in FIG. 6 exhibit such crystallizationpeaks, indicating that the glass compositions 25Na, 5Te, 5Ge, and 5Al donot exhibit very good thermal properties

In contrast, the DSC curves 70, 71, and 72 shown in FIG. 7,corresponding respectively to the glasses 10Al5Ge, 10Al and 5Al5Gelisted in Table 1, do not show exothermic peaks and are indicative ofexcellent thermal properties. The lack of strong exothermic peaks in anyof these curves indicates that these glasses do not crystallize whenheated. Such properties are highly desirable when the glasses are to bedrawn into fibers. The characteristic temperatures for all glasses arereported in Table 2. Except for the glass compositions 25Na, 5Te, 5Ge,and 5Al, all the other glass compositions described in Table 1 lack acrystallization signature upon heating in DSC curves. This illustratesthe excellent thermal properties of the glass compositions of thepresent invention.

TABLE 2 Characteristic temperatures for all glasses. Tg (glass) − GlassTg (° C.) Tx (° C.) Tc (° C.) Tm (° C.) Tg (25 Na) 25Na 279 365    s 397522 0 5Te 292 370    s 412 553 13 5Ge 310 380    vs 432  541 31 7Ge 323″ ″  s 533 44 5Ge2Te 314 ″ ″ 534 35 5Al 325 398    vs 437  537 46 7Al342 ″ ″ 549 63 10Al 362 ″ ″ — 83 5Al2Te 334 ″ ″ 546 55 5Al5Ge 344 ″ ″ vs520 65 10Al5Ge 375 ″ ″  s 551 96 15K25W 352 ″ ″ vs 537 — Tg: glasstransition temperature; Tx: onset crystallization temperature; Tc:crystallization peak temperature; Tm: melting point temperature ifvisible under 800° C. (s = smooth; vs = very smooth).Chemical Durability

For optical communication applications, EDFAs must resist the naturalcorrosion exercised by the environment for several decades. The chemicaldurability of the glasses was tested in boiling water. The faster theglass looses weight in boiling water, the worse is its chemicaldurability. FIGS. 8, 9 and 10 represent the weight loss per unit ofsurface as a function of immersion time in boiling water for all theglass compositions listed in Table 1. For clarity, these curves havebeen split into three figures. The glasses (25Na 80, 7Ge 81, 5Ge2Te 82,5Te 83, 5Ge 84, and 5Al 85) with the highest weight losses are presentedin FIG. 8, those glasses (5Al 85, 5Al2Te 86, 5Al5Ge 87, 7Al 88, and15K25W 89) with medium weight losses in FIG. 9, and those glasses(15K25W 89, 10Al5Ge 90, and 10Al 91) with the lowest weight losses inFIG. 10. For ease of comparison, the curve obtained for the glass 5Al 85is repeated in FIG. 9. Likewise that of the glass 15K25W 89 is repeatedin FIG. 10.

The glass composition with the highest alkali metal oxide content, 25Na,has the lowest chemical durability. After one hour in boiling water, theglass lost 1.74 mg/mm². Glasses containing high alkali metal oxideconcentrations have the tendency to exhibit a higher decomposition ratein water. Alkali metal ions favor the penetration of water in the glassthat leads to a decomposition of the glass network. Therefore a lowerconcentration of Na₂O in the glass results in a better chemicaldurability. As shown in FIG. 10, when the Al₂O₃ concentration is 10%,glasses 10Al 91 and 10Al5Ge 90 show good chemical durability. Theirrespective weight losses after one hour of immersion in boiling waterare 0.00255 mg/mm² and 0.00485 mg/mm².

Density and Refractive Index

Table 3 summarizes the weight density and the refractive index measuredat several wavelengths (633, 830, 1307, and 1550 nm) for all the glasscompositions reported in Table 1. The refractive index increases withthe density of the glass. The introduction of Al₂O₃ tends to decreasethe density of the glass and consequently its refractive index. When themodifier Na₂O is replaced by the intermediate Al₂O₃ the number ofnon-bridging oxygen decreases. In oxide glasses, the ionic refractivityof bridging oxygen is smaller than the ionic refractivity ofnon-bridging oxygen. So the replacement of Na₂O by Al₂O₃ leads to adecrease of the refractive index. Another contribution might beattributed to an increased polarizability of Na⁺ ions versus Al³⁺because their ionic radius is larger.

The density of the glass decreases when Na₂O is replaced by Al₂O₃. Whilethe glass modifier Na₂O fills the cavities of the preexisting glassstructure, the intermediate Al₂O₃ participates and changes the networkof the glass. Hence, Al₂O₃ expands the volume of the glass network.Since Na⁺ and Al³⁺ have the same weight, the volume increase associatedwith the addition of Al₂O₃ translates into a decrease in density of theglass.

As shown in Table 3, the introduction of Ge or more Te in the glassincreases the refractive index. Ge and Te, like other heavy atomsincrease the refractive index. This effect is the strongest with Te,which is heavier, larger, and consequently leads to a higherpolarizability compared with Ge. GeO₂ and TeO₂ as network formers modifythe structure of the glass and could expand its volume like Al₂O₃.However, since they are heavy atoms, the density of the glass isincreased, when used instead of Na₂O.

TABLE 3 Glass density 25° C. (+/− 0.01 g/cm³) and refractive indexmeasured at several wavelengths in various glasses. n at n at n at n atGlass d (g/cm³) 633 nm 830 nm 1307 nm 1550 nm 25 Na 4.26 1.8406 1.82221.8066 1.801 5 Te 4.45 1.8888 1.8682 1.8509 1.8449 5 Ge 4.4 1.86671.8477 1.8313 1.8258 7 Ge 4.46 1.8728 1.8535 1.8372 1.8315 5 Ge 2 Te4.47 1.893 1.8737 1.8584 1.8506 5 Al 4.24 1.8433 1.8254 1.8107 1.8047 7Al 4.26 1.8384 1.8213 1.8059 1.801 10 Al 4.18 1.8121 1.796 1.7822 1.77765 Al 2 Te 4.31 1.8594 1.8399 1.8255 1.8202 5 Al 5 Ge 4.34 1.8508 1.83291.8178 1.8127 10 Al 5 Ge 4.3 1.8386 1.8221 1.8076 1.8023 15 K 25 W 5.341.9949 1.9697 1.9481 1.9432Phonon Energy

Boron oxide is introduced into tellurite oxide glasses to increase thephonon energy of the lattice, while maintaining good chemical durabilityof the glass. Transmission spectra in the infrared were measured usinginfra-red spectroscopy to verify this effect. The glasses tested can beclassified in four groups: 1) alkali-boro-tellurite glasses; 2)alkali-boro-tellurite glasses containing Al₂O₃ or GeO₂; 3) telluriteglasses containing some tungsten oxide; and 4) pure alkali-telluriteglasses. Table 4 summarizes the glasses tested and the measured phononenergies.

TABLE 4 Absorption wave number (cm⁻¹) from the transmission spectraComposition of the glass (% mol) - V: curve in V shape, W: curve in Wshape, Sh: shoulder, FSh: flat 1_(WT) Er₂O₃ shoulder 30 Na₂O-10 B₂O₃-60TeO₂ 1321.9 1014.4 941.1 759.0 697.0 (V) (W) (W) (V) (Sh) 25 Na₂O-15B₂O₃-60 TeO₂ 1332.8 1258.7 1029.2 935.7 763.2 682.6 (V) (Sh) (Sh) (V)(FSh) (V) 20 Na₂O-20 B₂O₃-60 TeO₂ 1337.5 1263.1 1046.6 938.9 766.8 695.7(V) (FSh) (Sh) (V) (Sh) (Sh) 20 Na₂O-15 B₂O₃-65 TeO₂ 1331.9 1258.71035.5 933.3 764.5 689.0 (V) (Sh) (Sh) (V) (Sh) (Sh) 20 Na₂O-5 Al₂O₃-15B₂O₃-60 TeO₂ 1340.0 1245.2 1020.0 910.0 761.4 715.7 (W) (W) (FSh) (FSh)(Sh) (V) 20 Na₂O-5 GeO₂-15 B₂O₃-60 TeO₂ 1334.2 1255.3 1035.5 934.4 764.5685.7 (V) (Sh) (Sh) (V) (Sh) (Sh) 20 K₂O-10 WO₃-10 B₂O₃-60 TeO₂ 1335.11248.8 1059.1 *916.8  848.7 785.9 690.8 (W) (W) (Sh) (V) (FSh) (Sh) (Sh)15 K₂O-25 WO₃-60 TeO₂ *928.6  845.5 773.0 682.1 (V) (FSh) (Sh) (Sh) 35Na₂O-65 TeO₂ 756.4 (V) 30 Na₂O-70 TeO₂ 757.9 688.5 (Sh) (Sh) Componentthe most responsible B₂O₃ B₂O₃ B₂O₃ B₂O₃ WO₃ TeO₂ TeO₂ Vibration bondattribution ═B—O≡ ═B—O—B≡ ═B—O—B≡ O—Te—O O—Te—O s = stretching, s-s =symmetry or B—O—B or or [WO₄] or or stretching, as-s = asymmetrystretching, O—B—O (s) ═B—O— ═B—O— (g) Te—O⁻ Te—O⁻ b = bending, g = group(as-s) (as-s) (as-s) (as) (as) Other responsible component *WO₃ B₂O₃could influence ═B—O—B═ (b)

FIG. 11 shows the transmission spectra of two alkali-tellurite glasses35 Na₂O−65 TeO₂−1_(WT) Er₂O₃ 100 and 30 Na₂O−70 TeO₂ −Er ₂O₃ 101. Thisfigure clearly illustrates that the highest phonon energy of TeO₂ isaround 757 cm⁻¹. The alkaline oxide, as a modifier, has no influence onthe phonon energy of the lattice of the glass. This absorption around757 cm⁻¹ can be attributed to the asymmetric vibration of the O—Te—O orO—Te—O⁻bonds.

FIG. 12 shows the transmission spectra of alkali-boro-tellurite glasses30 Na₂O−10 B₂O₃−60 TeO₂−1_(WT) Er₂O₃ 110, 20 Na₂O−15 B₂O₃−65 TeO₂−1_(WT)Er₂O₃ 111, 25 Na₂O−15 B₂O₃−60 TeO₂−1_(WT) Er₂O₃ 112, and 20 Na₂O−20B₂O₃−60 TeO₂−1_(WT) Er₂O₃ 113. As expected, the introduction of B₂O₃intotellurite glasses increases the phonon energy significantly up to 1337cm⁻¹ as also reported in Table 4. This phonon energy can be attributedto the asymmetric stretching of the bonds ═B—O≡ or O—B—O. Note that theshapes of the spectra shown in FIG. 12 are nearly identical for all fourglasses. A small difference is in the lack of a shoulder near 1260 cm⁻¹in glass 110 containing 10% of B₂O₃. Since this band is due to the B—O—Bstretching, we suspect that 10% of B₂O₃ is not enough to have asignificant proportion of two boron atoms connected to the same oxygen,if the glass is homogeneous.

FIG. 12 also shows that the addition of boron oxide to the telluriteglasses leads to the appearance of another broad absorption band around937 cm⁻¹ with a shoulder at around 1033 cm⁻¹. These two bands can beattributed to the asymmetric stretching of ═B—O—B≡ or ═B—O— bonds. Asexpected the absorption bands due to TeO₂ at around 764 and 692 cm⁻¹ arestill observable.

The transmission spectra of glasses with compositions 20 Na₂O−5 Al₂O₃−15B₂O₃−60 TeO₂−1 wt. % Er₂O₃ 120 and 20 Na₂O−GeO₂−15 B₂O₃−60 TeO₂−1 wt. %Er₂O₃ 121 shown in FIG. 13 are very similar to those in FIG. 12. Theaddition of 5% of germanium oxide doesn't seem to influence the spectrumin one-way or another. In contrast, when Al₂O₃ is added toalkali-boro-tellurite glasses the transmission spectrum changes as shownin FIG. 13.

In alkali-tellurite glasses containing tungsten oxide such as in glasseswith compositions 15 K₂O−25 WO₃−60 TeO₂−1_(WT) Er₂O₃ 130 and 20 K₂O−10WO₃−10 B₂O₃−60 TeO₂−1_(WT) Er₂O₃ 131, two additional absorption bandsare observed respectively at 929 and 846 cm⁻¹ as illustrated in FIG. 14.The two bands 140 and 141 are attributed to the tungsten oxide and moreparticularly to the vibrations of the WO₄ tetrahedrons. When B₂O₃ isadded to a tungstate alkali-tellurite glass, the absorption band at 1249cm⁻¹ typical of the B—O—B stretching mode is well defined. Thisindicates that the proportion of two boron atoms connected to the sameoxygen is important and that consequently WO₃ and B₂O₃ can not be mixedwell in this particular glass composition.

As a proof, when the glass was cast at low cooling rate, the glass hadpoor optical transparency, indicative of phase separation and poorhomogeneity.

Spectral properties

The optical properties (absorption and emission) of the erbium ions thatwere doped into the different glasses are summarized in Table 5. Thesecond column of the table indicates the concentration of erbium ions.This concentration varies with the glass composition because theconstant amount of erbium oxide that was incorporated into the differentglasses was measured in wt %. The third column (∫σ_(a)(λ)dλ in cm²)describes the total absorption cross section of the 1550 nm band. Thefourth column gives the absorption bandwidth (Δλ_(a) in nm). The fifthand sixth columns describe the total emission cross section of the 1550nm band (∫σ_(e)(λ)dλ in cm²), and the emission bandwidth (Δλ_(e) in nm),respectively. The last column lists the measured lifetime (τ_(meas). inms) of the ⁴I_(13/2) excited state.

TABLE 5 Optical properties of Er³⁺ ions contained in various glasses.τ_(meas.) Er³⁺ ∫σ_(a)(λ)dλ ∫σ_(e)(λ)dλ Δλ_(e meas.) (+/− 0.1 ms) atGlass (10²⁰ ions/cm³) (10⁻¹⁹ cm²) Δλ_(a) (nm) (10⁻¹⁹ cm²) Δλ_(e) (nm)(nm) 1535 nm 25 Na 1.328 4.47 57.28 4.53 54.47 54.30 3.17 5 Te 1.3874.84 59.33 5.08 57.86 56.01 2.98 5 Ge 1.372 5.07 59.03 5.28 57.40 55.802.97 7 Ge 1.389 4.83 60.45 5.16 59.91 57.48 3.26 5 Ge 2 Te 1.392 4.6860.06 4.62 58.66 55.22 3.41 5 Al 1.32 4.74 59.32 4.91 57.48 56.21 2.93 7Al 1.327 4.39 60.52 4.59 58.88 55.97 2.81 10 Al 1.301 4.68 62.84 4.9461.90 61.37 3.07 5 Al 2 Te 1.342 4.78 60.49 4.93 59.02 59.25 2.96 5 Al 5Ge 1.352 4.87 61.68 5.13 60.04 57.06 3.20 10 Al 5 Ge 1.339 4.93 66.815.32 66.23 61.99 2.05 15 K 25 W 1.665 5.12 61.29 5.41 60.12 55.75 3.66Gain Properties

To evaluate the optical gain properties of Er³⁺ doped boro-telluriteglasses for use in optical amplifiers and lasers, optical fibers werefabricated from these glasses and tested. The preform for the fiber wasfabricated from the following glasses: for the core a glass withcomposition 60TeO₂+15B₂O₃+10Al₂O₃+15Na₂O+0.5 wt. % Er₂O₃ was used, andfor the cladding a glass with composition57.75TeO₂+15B₂O₃+10Al₂O₃+15Na₂O ZnO. At the wavelength of 1550 nm, therefractive index of the core and cladding were n=1.7834 and n=1.7738,respectively. The fiber was drawn from the preform using standard fiberpulling techniques.

FIG. 15 shows the gain 150 and noise FIG. 151 measured in a 15 cm-longtellurite fiber for an input signal at 1535 nm and as a function of thepower of the pump with wavelength 976 nm. A gain of 12 dB was achievedat a pumping power of 110mw and the corresponding noise figure wasaround 5 dB. The gain curve indicates that the saturation is notreached. The gain can be further raised by increasing the pumping poweror by increasing the fiber length.

FIG. 16 is the gain spectrum 160 measured in the same tellurite fiber ata pumping power of 112 mW. A maximum gain of 13.5 dB is achieved at 1533nm and a gain of 2 dB is measured at longer wavelength near 1600 nm.These results show that these new glasses can provide gain over a broadspectrum, especially at longer wavelengths

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

1. A boro-tellurite glass composition comprising the followingingredients: TeO₂ from 50 to 70 mole percent, B₂O₃ from 5 to 22 molepercent, R₂O from 5 to 25 mole percent, MO from 0 to 15 mole percent,A₂O₃ from 5 to 18 mole percent, GeO₂ from 0 to 7 mole percent, and L₂O₃from 0.25 to 10 weight percent, wherein R₂O is selected from the oxidesLi₂O, K₂O, Na₂O and mixtures thereof, MO is selected from the oxidesBaO, MgO, CaO, ZnO and mixtures thereof, A₂O₃ is selected from Al₂O₃,Y₂O₃ and mixtures thereof, and L₂O₃ is selected from rare earth oxidesEr₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixtures thereof.2. The boro-tellurite glass composition of claim 1, wherein thecomposition comprises TeO₂ from 55 to 65 mole percent, B₂O₃ from 10 to20 mole percent, A₂O₃ from 7 to 15 mole percent, a glass networkmodifier R₂O from 10 to 20 mole percent, a glass network modifier MOfrom 0 to 10 mole percent, GeO₂ from 0 to 5 mole percent and rare-earthdopant L₂O₃ from 0.25 to 6 weight percent.
 3. The boro-tellurite glassof claim 2, wherein A₂O₃ is 7 to 15 mole percent Al₂O₃.
 4. Theboro-tellurite glass of claim 2, wherein A₂O₃ is 10 to 15 mole percentAl₂O₃.
 5. The boro-tellurite glass composition of claim 2, wherein thecomposition comprises approximately 60 mole percent TeO₂, approximately15 mole percent B₂O₃, approximately 10 mole percent Al₂O₃, andapproximately 15 mole percent Na₂O.
 6. The boro-tellurite glasscomposition of claim 2, wherein the rare-earth dopant L₂O₃ comprisesapproximately 0.25 to 3 weight percent of Er₂O₃.
 7. The boro-telluriteglass composition of claim 2, wherein the rare-earth dopant L₂O₃comprises a mixture of approximately 0.25 to 5 weight percent of Er₂O₃and Yb₂O₃.
 8. The boro-tellurite glass composition of claim 7, whereinthe mixture comprises 0.25 −3 weight percent Er₂O₃ and 0.25 −3 weightpercent Yb₂O₃.
 9. The boro-tellurite glass composition of claim 2,wherein the rare-earth dopant L₂O₃ comprises approximately 0.25 to 3weight percent of Tm₂O₃.
 10. A boro-tellurite glass compositioncomprising the following ingredients: TeO₂ from 55 to 65 mole percent,B₂O₃ from 10 to 20 mole percent, Na₂O from 10 to 20 mole percent, MOfrom 0 to 10 mole percent, Al₂O₃ from 7 to 15 mole percent, GeO₂ from 0to 7 mole percent, and L₂O₃ from 0.25 to 6 weight percent, wherein MO isselected from the oxides BaO, MgO, CaO, ZnO and mixtures thereof, andL₂O₃ is selected from rare earth oxides Er₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃,CeO₂, Sm₂O₃ and Nd₂O₃ and mixtures thereof.
 11. The boro-tellurite glassof claim 10, comprising 10 to 15 mole percent Al₂O₃.
 12. Theboro-tellurite glass composition of claim 10, wherein the rare-earthdopant L₂O₃ comprises approximately 0.25 to 3 weight percent of Er₂O₃.13. The boro-tellurite glass composition of claim 10, wherein therare-earth dopant L₂O₃ comprises a mixture of approximately 0.25 to 5weight percent of Er₂O₃ and Yb₂O₃.
 14. The boro-tellurite glasscomposition of claim 10, wherein the mixture comprises 0.25 -3 weightpercent Er₂O₃ and 0.25 -3 weight percent Yb₂O₃.
 15. An optical fiber,comprising a core and a cladding formed of a glass having the followingingredients: TeO₂ from 50 to 70 mole percent, B₂O₃ from 5 to 22 molepercent, R₂O from 5 to 25 mole percent, MO from 0 to 20 mole percent,A₂O₃ from 5 to 18 mole percent, GeO₂ from 0 to 7 mole percent, whereinR₂O is selected from the oxides Li₂O, K₂O, Na₂O and mixtures thereof, MOis selected from the oxides BaO, MgO, CaO, ZnO and mixtures thereof,A₂O₃ is selected from Al₂O₃, Y₂O₃ and mixtures thereof, and said corefurther comprising, Rare-earth dopant L₂O₃ from 0.25 to 10 weightpercent, wherein L₂O₃ is selected from rare earth oxides Er₂O₃, Yb₂O₃,Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixtures thereof.
 16. Theoptical fiber of claim 15, wherein the composition comprises TeO₂ from55 to 65 mole percent, B₂O₃ from 10 to 20 mole percent, A₂O₃ from 7 to15 mole percent, a glass network modifier R₂O from 10 to 20 molepercent, a glass network modifier MO from 0 to 10 mole percent, GeO₂from 0 to 5 mole percent and rare-earth dopant L₂O₃ from 0.25 to 6weight percent.
 17. The optical fiber of claim 16, wherein A₂O₃ is 10 to15 mole percent Al₂O₃.
 18. The optical fiber of claim 16, wherein therare-earth dopant L₂O₃ comprises approximately 0.25 to 3 weight percentof Er₂O₃.
 19. The optical fiber of claim 16, wherein the rare-earthdopant L₂O₃ comprises a mixture of approximately 0.25 to 5 weightpercent of Er₂O₃ and Yb₂O₃.
 20. An erbium doped fiber amplifier (EDFA),comprising: A fiber having a core and a cladding formed from a glasshaving the following ingredients: TeO₂ from 50 to 70 mole percent, B₂O₃from 8 to 22 mole percent, R₂₀ from 5 to 20 mole percent; MO from 0 to20 mole percent, A₂O₃ from 7 to 18 mole percent, GeO₂ from 0 to 7 molepercent, wherein R₂O is selected from the oxides Li₂O, K₂O, Na₂O andmixtures thereof, MO is selected from the oxides BaO, MgO, CaO, ZnO andmixtures thereof, A₂O₃ is selected from Al₂O₃, Y₂O₃ and mixturesthereof, said core further comprising rare-earth dopant L₂O₃ from 0.25to 10 weight percent, wherein L₂O₃ is selected from rare earth oxidesEr₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ mixtures thereof; andA 980 nm optical pump configured to pump the ionic energy levels of therare-earth dopant in said fiber to produce stimulated emission andamplification of an input signal propagating through said fiber.
 21. TheEDFA of claim 20, wherein the composition comprises TeO₂ from 55 to 65mole percent, B₂O₃ from 10 to 20 mole percent, A₂O₃ from 7 to 15 molepercent, a glass network modifier R₂O from 10 to 20 mole percent, aglass network modifier MO from 0 to 10 mole percent, GeO₂ from 0 to 5mole percent and rare-earth dopant L₂O₃ from 0.25 to 6 weight percent.22. The EDFA of claim 21, wherein A₂O₃ is 10 to 15 mole percent Al₂O₃.23. The EDFA of claim 21, wherein the rare-earth dopant L₂O₃ comprisesapproximately 0.25 to 3 weight percent of Er₂O₃.
 24. The EDFA of claim21, wherein the rare-earth dopantL₂O₃ comprises a mixture ofapproximately 0.25 to 5 weight percent of Er₂O₃ and Yb₂O₃.
 25. The EDFAof claim 21, wherein optical pump comprises a multi-mode pump and a pumpcoupler.
 26. The EDFA of claim 25, wherein the pump coupler comprises aTIR coupler.
 27. A boro-tellurite glass composition comprising thefollowing ingredients: TeO₂ from 50 to 70 mole percent, B₂O₃ from 5 to22 mole percent, R₂O from 5 to 25 mole percent, MO from 0 to 15 molepercent, A₂O₃ from 0 to 18 mole percent, GeO₂ from 0 to 7 mole percent,and L₂O₃ from 0.25 to 10 weight percent, wherein R₂O is selected fromthe oxides Li₂O, K₂O, Na₂O and mixtures thereof, MO is selected from theoxides BaO, MgO, CaO, ZnO and mixtures thereof, A₂O₃ is selected fromAl₂O₃, Y₂O₃ and mixtures thereof, and L₂O₃ is selected from rare earthoxides Er₂O₃, Yb₂O₃, Tm₂O₃, Tb₂O₃, CeO₂, Sm₂O₃ and Nd₂O₃ and mixturesthereof.
 28. The boro-tellurite glass of claim 27, wherein A₂O₃ is 5 to15 mole percent Al₂O₃.
 29. The boro-tellurite glass of claim 27, whereinA₂O₃ is 10 to 15 mole percent Al₂O₃.